In the typical RIM operation, a mold cavity formed by at least two mating mold parts is filled with reactive chemicals that are mixed and injected at high pressure into the mold cavity, wherein an exothermic polymerization reaction substantially increases the pressure within the cavity. During the reaction process, it is important to clamp the mold parts firmly together to prevent the material being molded from escaping at the junctures between the mold parts. The RIM of large products requires tremendous compression forces over a comparatively large area, such that conventional presses used for high tonnage RIM operations tend to warp or deform during the molding process. Although the press beds between which comparatively large mold parts are pressed comprise heavy rigid steel structures, it has not been economically feasible to provide such presses that will not deform. In consequence, the force applied to clamp the mold parts together is distributed unevenly over the area of the mold, enabling the extrusion of pressurized reacting chemicals through tiny spacings at the mold junctures. Such spacings on the order of a thousandth of an inch are significant, may result in improperly formed molded products, and in any event necessitate an additional operation to remove flash from the molded product.
Upon completion of the RIM process, high pressure must often be applied to "strip" the mold parts from the molded product, depending upon the latter's size and configuration.
Although the prior art relating to molding presses is extensive, very little of that art known to applicant is concerned with the problem of preventing or compensating for deformation of the press components. The Hammon U.S. Pat. No. 4,304,540, is typical of a conventional type of press that ignores the deformation problem and is thus limited to the molding of small products involving comparatively low pressure applications. The Hammon press provides stress or clamping rods 18 mounted at the corners of fixed and movable press beds or mold supports 12 and 24 respectively. A locking assembly 22 carried by the movable bed 24 clamps the serrations 21 of each rod 18 to lock the movable bed 24 adjacent to a molding position prior to application of a clamping force. Thereafter pressure is applied to the upper sides of pistons 57, FIG. 1, which are secured to the rod 18 to pull the latter and bed 24 downwardly in a clamping action to the molding position. After the molding operation, pressure is applied to the lower ends of pistons 57 to effect a stripping action by pushing the rods 18 upwardly.
Such a press is suitable for use only with comparatively small molds because under extremely high tonnage force, in addition to deformation of other press components, the locking rods 18 are stretched, usually non-uniformly. Although the corner portions of the beds 12 and 24 are tightly clamped together, their central portions when subjected to high tonnage molding conditions are insufficiently clamped to the extent that the high pressure moldable products extrude from the mold as flashing.
The Quere U.S. Pat. No. 2,916,768, recognizes the problem of deformation and the possibility of improperly aligned press components. It provides for independent adjustment of the corner mounted stress rods 5 and for the use of different pressures in the actuating cylinders 3 to compensate for such deformation. Such a mold requires sophisticated controls and at best can only minimize deformation when the press is used with comparatively small molds. Even if the clamping forces at the corners of the mold are equalized, the mold will still be subject to the disadvantages of the corner mounted clamping devices used by Hammon.
One type of press known to the art is concerned with the provision of a uniform distribution of molding force over the area of a mold. Such presses, variously known as bladder or pillow type presses, provide a high pressure chamber having a movable and usually deformable wall coextensive with a movable mold plate and deformable against the latter to clamp it toward a fixed mold plate during a molding operation. Typically high speed means are also provided for moving the movable plate and high pressure chamber in unison to and from a mold closed position whereat the movable mold plate is adjacent to the fixed mold plate and in position to carry out the molding operation upon the injection of pressurized fluid into the high pressure chamber.
Such presses are only suitable for molding products having comparatively small surface areas requiring a comparatively small total molding force, wherein deformation of the press components is not a problem and high pressure stripping is not required. The deformable wall of the high pressure chamber can only exert a unidirectional molding force and is thus not suitable for effecting high pressure stripping.
A typical pillow or bladder type press is disclosed in the Descrovi U.S. Pat. No. 4,247,278, which recognizes the problem of deformation of the mold plates and provides a fluid pressurized cylinder 77 having an axial end wall 76, FIGS. 1, 2, or 216, FIGS. 3, 4, sufficiently thin and flexible to conform to deformation of an adjacent mold plate when the cylinder 77 is pressurized during a molding operation. Descrovi et al, like other pillow or bladder type patents, is not suitable for high tonnage operation. At the outset, it does not enable high pressure stripping by the same pressure exerting system that provides the mold closing pressure. Also, the area of the deformable walls 76 and 216 must be strictly limited. Otherwise these walls will be ruptured if subjected to the high pressure RIM of a large product. The deformable wall must be sufficiently thin to conform to deflection of the adjacent mold carrier and must be sufficiently thick to prevent its destruction by the pressure within cylinder 77. Accordingly bladder or pillow type presses such as Descrovi et al must be operated within a comparatively limited range of clamping pressure.